425 lines
11 KiB
C
425 lines
11 KiB
C
/* ehopt.c--optimize gcc exception frame information.
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Copyright (C) 1998 Free Software Foundation, Inc.
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Written by Ian Lance Taylor <ian@cygnus.com>.
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This file is part of GAS, the GNU Assembler.
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GAS is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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GAS is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GAS; see the file COPYING. If not, write to the Free
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Software Foundation, 59 Temple Place - Suite 330, Boston, MA
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02111-1307, USA. */
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#include "as.h"
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#include "subsegs.h"
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/* We include this ELF file, even though we may not be assembling for
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ELF, since the exception frame information is always in a format
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derived from DWARF. */
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#include "elf/dwarf2.h"
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/* Try to optimize gcc 2.8 exception frame information.
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Exception frame information is emitted for every function in the
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.eh_frame section. Simple information for a function with no
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exceptions looks like this:
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__FRAME_BEGIN__:
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.4byte .LLCIE1 / Length of Common Information Entry
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.LSCIE1:
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.4byte 0x0 / CIE Identifier Tag
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.byte 0x1 / CIE Version
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.byte 0x0 / CIE Augmentation (none)
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.byte 0x1 / ULEB128 0x1 (CIE Code Alignment Factor)
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.byte 0x7c / SLEB128 -4 (CIE Data Alignment Factor)
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.byte 0x8 / CIE RA Column
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.byte 0xc / DW_CFA_def_cfa
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.byte 0x4 / ULEB128 0x4
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.byte 0x4 / ULEB128 0x4
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.byte 0x88 / DW_CFA_offset, column 0x8
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.byte 0x1 / ULEB128 0x1
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.align 4
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.LECIE1:
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.set .LLCIE1,.LECIE1-.LSCIE1 / CIE Length Symbol
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.4byte .LLFDE1 / FDE Length
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.LSFDE1:
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.4byte .LSFDE1-__FRAME_BEGIN__ / FDE CIE offset
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.4byte .LFB1 / FDE initial location
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.4byte .LFE1-.LFB1 / FDE address range
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.byte 0x4 / DW_CFA_advance_loc4
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.4byte .LCFI0-.LFB1
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.byte 0xe / DW_CFA_def_cfa_offset
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.byte 0x8 / ULEB128 0x8
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.byte 0x85 / DW_CFA_offset, column 0x5
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.byte 0x2 / ULEB128 0x2
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.byte 0x4 / DW_CFA_advance_loc4
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.4byte .LCFI1-.LCFI0
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.byte 0xd / DW_CFA_def_cfa_register
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.byte 0x5 / ULEB128 0x5
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.byte 0x4 / DW_CFA_advance_loc4
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.4byte .LCFI2-.LCFI1
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.byte 0x2e / DW_CFA_GNU_args_size
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.byte 0x4 / ULEB128 0x4
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.byte 0x4 / DW_CFA_advance_loc4
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.4byte .LCFI3-.LCFI2
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.byte 0x2e / DW_CFA_GNU_args_size
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.byte 0x0 / ULEB128 0x0
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.align 4
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.LEFDE1:
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.set .LLFDE1,.LEFDE1-.LSFDE1 / FDE Length Symbol
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The immediate issue we can address in the assembler is the
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DW_CFA_advance_loc4 followed by a four byte value. The value is
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the difference of two addresses in the function. Since gcc does
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not know this value, it always uses four bytes. We will know the
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value at the end of assembly, so we can do better. */
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static int eh_frame_code_alignment PARAMS ((void));
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/* Get the code alignment factor from the CIE. */
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static int
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eh_frame_code_alignment ()
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{
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static int code_alignment;
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segT current_seg;
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subsegT current_subseg;
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fragS *f;
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fixS *fix;
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int offset;
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int eh_state;
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if (code_alignment != 0)
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return code_alignment;
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/* We should find the CIE at the start of the .eh_frame section. */
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current_seg = now_seg;
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current_subseg = now_subseg;
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subseg_new (".eh_frame", 0);
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f = seg_info (now_seg)->frchainP->frch_root;
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fix = seg_info (now_seg)->frchainP->fix_root;
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subseg_set (current_seg, current_subseg);
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/* Look through the frags of the section to find the code alignment. */
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/* First make sure that the CIE Identifier Tag is 0. */
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offset = 4;
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while (f != NULL && offset >= f->fr_fix)
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{
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offset -= f->fr_fix;
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f = f->fr_next;
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}
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if (f == NULL
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|| f->fr_fix - offset < 4
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|| f->fr_literal[offset] != 0
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|| f->fr_literal[offset + 1] != 0
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|| f->fr_literal[offset + 2] != 0
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|| f->fr_literal[offset + 3] != 0)
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{
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code_alignment = -1;
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return -1;
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}
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/* Next make sure the CIE version number is 1. */
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offset += 4;
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while (f != NULL && offset >= f->fr_fix)
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{
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offset -= f->fr_fix;
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f = f->fr_next;
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}
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if (f == NULL
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|| f->fr_fix - offset < 1
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|| f->fr_literal[offset] != 1)
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{
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code_alignment = -1;
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return -1;
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}
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/* Skip the augmentation (a null terminated string). */
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++offset;
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eh_state = 0;
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while (1)
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{
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while (f != NULL && offset >= f->fr_fix)
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{
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offset -= f->fr_fix;
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f = f->fr_next;
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}
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if (f == NULL)
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{
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code_alignment = -1;
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return -1;
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}
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while (offset < f->fr_fix && f->fr_literal[offset] != '\0')
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{
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switch (eh_state)
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{
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case 0:
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if (f->fr_literal[offset] == 'e')
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eh_state = 1;
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break;
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case 1:
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if (f->fr_literal[offset] == 'h')
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eh_state = 2;
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break;
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default:
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eh_state = 3;
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break;
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}
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++offset;
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}
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if (offset < f->fr_fix)
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break;
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}
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++offset;
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while (f != NULL && offset >= f->fr_fix)
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{
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offset -= f->fr_fix;
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f = f->fr_next;
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}
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if (f == NULL)
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{
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code_alignment = -1;
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return -1;
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}
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/* If the augmentation field is "eh", then we have to skip a
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pointer. Unfortunately, we don't know how large it is. We find
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out by looking for a matching fixup. */
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if (eh_state == 2)
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{
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while (fix != NULL
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&& (fix->fx_frag != f || fix->fx_where != offset))
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fix = fix->fx_next;
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if (fix == NULL)
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offset += 4;
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else
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offset += fix->fx_size;
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while (f != NULL && offset >= f->fr_fix)
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{
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offset -= f->fr_fix;
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f = f->fr_next;
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}
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if (f == NULL)
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{
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code_alignment = -1;
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return -1;
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}
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}
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/* We're now at the code alignment factor, which is a ULEB128. If
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it isn't a single byte, forget it. */
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code_alignment = f->fr_literal[offset] & 0xff;
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if ((code_alignment & 0x80) != 0 || code_alignment == 0)
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{
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code_alignment = -1;
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return -1;
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}
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return code_alignment;
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}
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/* This function is called from emit_expr. It looks for cases which
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we can optimize.
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Rather than try to parse all this information as we read it, we
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look for a single byte DW_CFA_advance_loc4 followed by a 4 byte
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difference. We turn that into a rs_cfa_advance frag, and handle
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those frags at the end of the assembly. If the gcc output changes
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somewhat, this optimization may stop working.
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This function returns non-zero if it handled the expression and
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emit_expr should not do anything, or zero otherwise. It can also
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change *EXP and *PNBYTES. */
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int
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check_eh_frame (exp, pnbytes)
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expressionS *exp;
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unsigned int *pnbytes;
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{
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static int saw_advance_loc4;
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static fragS *loc4_frag;
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static int loc4_fix;
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if (flag_traditional_format)
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{
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/* Don't optimize. */
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}
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else if (strcmp (segment_name (now_seg), ".eh_frame") != 0)
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saw_advance_loc4 = 0;
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else if (*pnbytes == 1
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&& exp->X_op == O_constant
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&& exp->X_add_number == DW_CFA_advance_loc4)
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{
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/* This might be a DW_CFA_advance_loc4. Record the frag and the
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position within the frag, so that we can change it later. */
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saw_advance_loc4 = 1;
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frag_grow (1);
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loc4_frag = frag_now;
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loc4_fix = frag_now_fix ();
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}
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else if (saw_advance_loc4
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&& *pnbytes == 4
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&& exp->X_op == O_constant)
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{
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int ca;
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/* This is a case which we can optimize. The two symbols being
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subtracted were in the same frag and the expression was
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reduced to a constant. We can do the optimization entirely
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in this function. */
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saw_advance_loc4 = 0;
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ca = eh_frame_code_alignment ();
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if (ca < 0)
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{
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/* Don't optimize. */
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}
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else if (exp->X_add_number % ca == 0
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&& exp->X_add_number / ca < 0x40)
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{
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loc4_frag->fr_literal[loc4_fix]
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= DW_CFA_advance_loc | (exp->X_add_number / ca);
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/* No more bytes needed. */
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return 1;
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}
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else if (exp->X_add_number < 0x100)
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{
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loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc1;
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*pnbytes = 1;
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}
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else if (exp->X_add_number < 0x10000)
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{
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loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc2;
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*pnbytes = 2;
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}
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}
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else if (saw_advance_loc4
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&& *pnbytes == 4
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&& exp->X_op == O_subtract)
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{
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/* This is a case we can optimize. The expression was not
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reduced, so we can not finish the optimization until the end
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of the assembly. We set up a variant frag which we handle
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later. */
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saw_advance_loc4 = 0;
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frag_var (rs_cfa, 4, 0, 0, make_expr_symbol (exp),
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loc4_fix, (char *) loc4_frag);
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return 1;
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}
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else
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saw_advance_loc4 = 0;
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return 0;
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}
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/* The function estimates the size of a rs_cfa variant frag based on
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the current values of the symbols. It is called before the
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relaxation loop. We set fr_subtype to the expected length. */
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int
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eh_frame_estimate_size_before_relax (frag)
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fragS *frag;
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{
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int ca;
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offsetT diff;
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int ret;
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ca = eh_frame_code_alignment ();
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diff = resolve_symbol_value (frag->fr_symbol, 0);
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if (ca < 0)
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ret = 4;
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else if (diff % ca == 0 && diff / ca < 0x40)
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ret = 0;
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else if (diff < 0x100)
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ret = 1;
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else if (diff < 0x10000)
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ret = 2;
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else
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ret = 4;
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frag->fr_subtype = ret;
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return ret;
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}
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/* This function relaxes a rs_cfa variant frag based on the current
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values of the symbols. fr_subtype is the current length of the
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frag. This returns the change in frag length. */
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int
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eh_frame_relax_frag (frag)
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fragS *frag;
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{
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int oldsize, newsize;
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oldsize = frag->fr_subtype;
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newsize = eh_frame_estimate_size_before_relax (frag);
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return newsize - oldsize;
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}
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/* This function converts a rs_cfa variant frag into a normal fill
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frag. This is called after all relaxation has been done.
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fr_subtype will be the desired length of the frag. */
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void
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eh_frame_convert_frag (frag)
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fragS *frag;
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{
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offsetT diff;
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fragS *loc4_frag;
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int loc4_fix;
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loc4_frag = (fragS *) frag->fr_opcode;
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loc4_fix = (int) frag->fr_offset;
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diff = resolve_symbol_value (frag->fr_symbol, 1);
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if (frag->fr_subtype == 0)
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{
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int ca;
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ca = eh_frame_code_alignment ();
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assert (ca > 0 && diff % ca == 0 && diff / ca < 0x40);
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loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc | (diff / ca);
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}
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else if (frag->fr_subtype == 1)
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{
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assert (diff < 0x100);
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loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc1;
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frag->fr_literal[frag->fr_fix] = diff;
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}
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else if (frag->fr_subtype == 2)
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{
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assert (diff < 0x10000);
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loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc2;
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md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 2);
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}
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else
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md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4);
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frag->fr_fix += frag->fr_subtype;
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frag->fr_type = rs_fill;
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frag->fr_offset = 0;
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}
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